Coating composition for precoated steel sheet, precoated steel sheet, and process for preparing the same

Abstract

PCT No. PCT/JP98/00350 Sec. 371 Date Oct. 22, 1998 Sec. 102(e) Date Oct. 22, 1998 PCT Filed Jan. 28, 1998 PCT Pub. No. WO98/32808 PCT Pub. Date Jul. 30, 1998The invention relates to a thermosetting coating composition for precoated steel sheets which are required to have high hardness, good workability and stain resistance, to a precoated steel sheet having a film of the coating composition, and to a method for producing such precoated steel sheets. The coating composition for precoated steel sheets of the invention consists essentially of a polyol and contains a specific compound and a curing agent composed of polyisocyanate compounds added thereto, in which the curing agent is a mixture composed of a blocked polyisocyanate compound as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (a) and their derivatives, and a blocked polyisocyanate compound as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (b) and their derivatives: (a) 4,4'-diphenylmethane diisocyanate, isophorone diisocyanate, tolylene diisocyanate, xylylene diisocyanate, naphthalene-1,5-diisocyanate; (b) hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate. The coating composition forms a coating film having good workability, high hardness and good stain resistance.

What is claimed is: 
     
       1. A coating composition for precoated steel sheets, which consists essentially of a resin component comprising the following i), ii) and iii): i) from 1 to 15% by weight, relative to the solid resin content of the composition, of a polyester compound comprising main repeating units of a chemical formula: ##STR15## wherein n represents an integer of 1 or more; and R represents an alkylene group having 10 or less carbon atoms, or a 1,4-cyclohexylene-dimethylene group, or a neopentylene group, or a polyoxyalkylene group;   ii) a polyol except i); and   iii) a curing agent of a mixture composed of a blocked polyisocyanate compound (A) as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanate of the following group (a), and a blocked polyisocyanate compound (B) as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (b), in a ratio by weight in terms of the solid contents, (A)/(B), of from 70/30 to 30/70: (a) 4,4'-diphenylmethane diisocyanate, isophorone diisocyanate, tolylene diisocyanate, xylylene diisocyanate, naphthalene-1,5-diisocyanate;   (b) hexamethylene diiisocyanate, trimethylhexamethylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate, with the amount of the curing agent of iii) being so defined that the ratio by mol of the isocyanato groups, (NCO), in the curing agent to the total of the hydroxyl groups, (OH), in the polyester compound of i) and the polyol of (ii), (NCO)/(OH), is from 0.8 to 1.2.       
     
     
       2. A coating composition for precoated steel sheets, which consists essentially of a resin component comprising the following i), ii) and iii): i) from 1 to 18% by weight, relative to the solid resin content of the composition, of a polyester compound comprising main repeating units of a chemical formula: ##STR16## wherein n represents an integer of 1 or more; and R represents an alkylene group having 10 or less carbon atoms, or a 1,4-cyclohexylene-dimethylene group, or a neopentylene group, or a polyoxyalkylene group;   ii) a polyol except i); and   iii) a curing agent of a mixture composed of a blocked polyisocyanate compound (A) as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanate of the following group (a), and a blocked polyisocyanate compound (B) as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (b), in a ratio by weight in terms of the solid contents, (A)/(B). of from 70/30 to 30/70: (a) 4,4'-diphenylmethane diisocyanate, isophorone diisocyanate, tolylene diisocyanate, xylylene diisocyanate, naphtdalene-1,5-diisocyanate;   (b) hexamethylene diiisocyanate, trimethylhexamethylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate, with the amount of the curing agent of iii) being so defined that the ratio by mol of the isocyanato groups, (NCO), in the curing agent to the total of the hydroxyl groups, (OH), in the polyester compound of i) and the polyol of (ii), (NCO)/(OH), is from 0.8 to 1.2.       
     
     
       3. A coating composition for precoated steel sheets, which consists essentially of a resin component comprising the following i), ii) and iii): i) from 1 to 15% by weight, relative to the solid resin content of the composition, of a compound of a chemical formula:   HO--(CH.sub.2).sub.n --O--R--O--(CH.sub.2).sub.n --OH     wherein n represents an integer of from 2 to 10; and R represents ##STR17## ii) a polyol except i); and iii) a curing agent of a mixture composed of a blocked polyisocyanate compound (A) as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanate of the following group (a), and a blocked polyisocyanate compound (B) as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (b), in a ratio by weight in terms of the solid contents, (A)/(B), of from 70/30 to 30/70:   (a) 4,4 '-diphenylmethane diisocyanate, isophorone diisocyanate, tolylene diisocyanate, xylylene diisocyanate, naphthalene-1,5-diisocyanate;   (b) hexamethylene diiisocyanate, trimethylhexamethylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate, with the amount of the curing agent of iii) being so defined that the ratio by mol of the isocyanato groups, (NCO), in the curing agent to the total of the hydroxyl groups, (OH), in the polyester compound of i) and the polyol of (ii), (NCO)/(OH), is from 0.8 to 1.2.       
     
     
       4. The coating composition for precoated steel sheets as claimed in claim 1, 2, or 3, wherein the blocked polyisocyanate compound (A) as obtained by blocking the polyisocyanate compound selected from polyisocyanates of the group (a) is any of isocyanurates, adducts and prepolymers, and the blocked polyisocyanate compound (B) as obtained by blocking the polyisocyanate compound selected from polyisocyanates of the group (b) is any of biuret, isocyanurates, adducts and prepolymers. 
     
     
       5. The coating composition for precoated steel sheets as claimed in claim 1, 2 or 3, wherein the polyol except i) comprises an acrylic resin having 2 or more hydroxyl groups in one molecule and having a number-average molecular weight of from 1500 to 12000, and/or a polyester resin having 2 or more hydroxyl groups in one molecule and having a number-average molecular weight of from 1000 to 8000. 
     
     
       6. A precoated steel sheet having a coating film formed from a coating composition of claim 1, 2, or 3. 
     
     
       7. A precoated steel sheet, which has a prime coating film and a topcoating film formed on the chemically-treated surface of a zinc or zinc alloy plated steel sheet substrate and in which the topcoating film is formed from a coating composition of claim 1, 2, or 3. 
     
     
       8. The precoated steel sheet as claimed in claim 7, wherein the prime coating film has a breaking strength of not smaller than 50 kg/cm 2  and a breaking elongation of not smaller than 50%, when measured in the form of the film having a width of 5 mm and a thickness of 8±1 μm. 
     
     
       9. The precoated steel sheet as claimed in claim 8, wherein the prime coating film is formed from a coating composition consisting essentially of a bisphenol A-added polyester resin. 
     
     
       10. The precoated steel sheet as claimed in claim 7, wherein the prime coating film contains strontium chromate in an amount of from 0.06 to 0.26 g/m 2  of the film of 1 μm thick. 
     
     
       11. The precoated steel sheet as claimed in claim 10, wherein the prime coating film is formed from a coating composition consisting essentially of a polyester resin. 
     
     
       12. The precoated steel sheet as claimed in claim 7, wherein the prime coating film has a dry film thickness of from 2 to 12 μm. 
     
     
       13. The precoated steel sheet as claimed in claim 7, wherein the topcoating film contains wax in an amount of from 0.3 to 30 parts by weight, relative to 100 parts by weight of the solid content of the coating composition exclusive of wax. 
     
     
       14. The precoated steel sheet as claimed in claim 13, wherein the wax to be in the topcoating film has a melting point of not lower than 100° C. 
     
     
       15. The precoated steel sheet as claimed in claim 13, wherein the wax to be in the topcoating film is a mixture composed of polyethylene wax and polytetrafluoroethylene. 
     
     
       16. A method for producing precoated steel sheets, which comprises applying a prime coating composition onto the chemically-treated surface of a zinc or zinc alloy plated steel sheet, then baking it at a peak metal temperature of from 180 to 260° C., then applying thereto a topcoating composition of claim 1, 2, or 3, and thereafter baking it at a peak metal temperature of from 180 to 260° C.